Fail-to-neutral system and method for a toroidal traction drive automatic transmission

ABSTRACT

A fail-to-neutral diagnostic technique for a transmission that includes a variator may include monitoring a state of a pressure differential valve fluidly coupled to a high side pressure applied to at least one actuator coupled to at least one corresponding roller of the variator and also fluidly coupled to a low side pressure applied to the at least one actuator, determining from the state of the pressure differential valve a variator torque sign corresponding to whether torque transferred by the at least one roller is positive or negative, determining an expected variator torque sign based on current operating conditions of the transmission, and commanding the transmission to a true neutral condition if the determined variator torque sign is different from the expected variator torque sign.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 13/847,324 entitled “FAIL-TO-NEUTRAL SYSTEM AND METHOD FOR ATOROIDAL TRACTION DRIVE AUTOMATIC TRANSMISSION,” which was filed on Mar.19, 2013, which is a divisional application of U.S. application Ser. No.12/957,885, entitled “FAIL-TO-NEUTRAL SYSTEM AND METHOD FOR A TOROIDALTRACTION DRIVE AUTOMATION TRANSMISSION,” which was filed on Dec. 1,2010, and which claims priority to U.S. Provisional Patent ApplicationSer. No. 61/287,045, which was filed on Dec. 16, 2009, the entirety ofeach of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to toroidal traction driveautomatic transmissions including a variator, and more specifically tosystems and methods for automatically controlling the transmission to aneutral state upon detection of variator and/or clutch related failures.

BACKGROUND

Toroidal traction drive automatic transmissions may include a variator,one or more gear sets and a number of selectively engageable frictiondevices that cooperate together to transfer drive torque from a powerplant to one or more loads. It is desirable to monitor operation of oneor more of these devices and to command the transmission to a neutralstate upon detection of one or more specified faults or failureconditions.

SUMMARY

The present invention may comprise one or more of the features recitedin the attached claims, and/or one or more of the following features andcombinations thereof. A fail-to-neutral diagnostic method for atransmission including a variator may comprise monitoring a state of apressure differential valve fluidly coupled to a high side pressureapplied to at least one actuator coupled to at least one correspondingroller of the variator and also fluidly coupled to a low side pressureapplied to the at least one actuator, determining from the state of thepressure differential valve a variator torque sign corresponding towhether torque transferred by the at least one roller is positive ornegative, determining an expected variator torque sign based on currentoperating conditions of the transmission, and commanding thetransmission to a true neutral condition if the determined variatortorque sign is different from the expected variator torque sign.

The pressure differential valve may comprise a spool having one endfluidly coupled to the high-side pressure and an opposite end fluidlycoupled to the low-side pressure. The variator torque sign may have onevalue when the high-side pressure is sufficiently greater than thelow-side pressure and may have an opposite value when the low-sidepressure is sufficiently greater than the high-side pressure.Determining from the state of the pressure differential valve a variatortorque sign may comprise determining the variator torque sign based on aposition of the spool relative to the pressure differential valve and. Apressure switch may be fluidly coupled to the pressure differentialvalve, and may be configured to assume one state when the high-sidepressure is sufficiently greater than the low-side pressure to cause thespool to move to one extreme relative to the pressure differentialvalve, and to assume an opposite state when the low-side pressure issufficiently greater than the high-side pressure to cause the spool tomove to an opposite extreme relative to the pressure differential valve.Determining from the state of the pressure differential valve a variatortorque sign may comprise determining the variator torque sign from thestate of the pressure switch. The variator torque sign may have onevalue when the pressure switch has assumed the one state and may have anopposite value when the pressure switch has assumed the opposite state.The expected variator torque sign may have the one value if the pressureswitch is expected to be in the one state, and may have the oppositevalue if the pressure switch is expected to be in the opposite state.Commanding the transmission to a true neutral condition may comprisecommanding the transmission to the true neutral condition if thevariator torque sign is the one value and the expected variator torquesign is the opposite value, and if the variator torque sign is theopposite value and the expected variator torque sign is the one value.

The method may further comprise logging a fault code in a memory unit ifthe determined variator torque sign is different from the expectedvariator torque sign.

A fail-to-neutral diagnostic system for a transmission including avariator may comprise a pressure differential valve fluidly coupled to ahigh side pressure applied to at least one actuator coupled to thevariator and also fluidly coupled to a low side pressure applied to theat least one actuator, a pressure switch fluidly coupled to the pressuredifferential valve and configured to assume one of two opposite statesdepending upon which of the high-side and low-side pressures is greaterthan the other, and a control circuit. The control circuit may include amemory having instructions stored therein that are executable by thecontrol circuit to determine from current operating conditions of thetransmission an expected state of the pressure switch, to determine anactual state of the pressure switch and to command the transmission to atrue neutral condition if the actual state of the pressure is differentfrom the expected state of the pressure switch.

The pressure switch may be configured to assume one state if thehigh-side pressure is sufficiently greater than the low-side pressureand to assume an opposite state if the low-side pressure is sufficientlygreater than the high-side pressure. The instructions stored in thememory may include instructions that are executable by the controlcircuit to determine that the expected state of the pressure switch isthe one state if the high-side pressure is expected to be sufficientlygreater than the low-side pressure and is the opposite state if thelow-side pressure is expected to be sufficiently greater than thehigh-side pressure. The instructions stored in the memory may furtherinclude instructions that are executable by the control circuit tocommand the neutral state if the pressure switch has assumed the onestate and the expected state of the pressure switch is the oppositestate, and to command the neutral state if the pressure switch hasassumed the opposite state and the expected state of the pressure switchis the one state. The instructions stored in the memory may furtherinclude instructions that are executable by the control circuit to log afault code in a memory unit if the determined actual state of thepressure switch is different from the expected state of the pressureswitch.

A fail-to-neutral diagnostic method for a transmission including avariator may comprise monitoring operating states of a plurality ofclutch control valves each fluidly coupled to a corresponding gearengagement clutch of the transmission, determining expected operatingstates of each of the plurality of clutch control valves based oncurrent operating conditions of the transmission, and forcing thetransmission to a true neutral condition if a monitored operating stateof at least one of the plurality of clutch control valves is differentfrom a corresponding one of the expected operating states.

The transmission may have a plurality of operating modes each operablein a different road speed ranges of a vehicle carrying the transmission.Determining expected operating states of each of the plurality of clutchcontrol valves may comprise determining which of the plurality ofoperating modes the transmission is currently operating in, anddetermining the expected operating states of each of the plurality ofclutch control valves for the current operating mode of the transmissionbased on the current operating conditions of the transmission. Themethod may further comprise a plurality of pressure switches eachfluidly coupled to a different one of the plurality of clutch controlvalves and each configured to assume an operating state depending uponthe operating state of a corresponding one of the plurality of clutchcontrol valves. Monitoring operating states of a plurality of clutchcontrol valves may comprise monitoring operating states of each of theplurality of pressure switches. Determining expected operating states ofeach of the plurality of clutch control valves may comprise determiningexpected operating states of each of the plurality of pressure switches.

The method may further comprise logging a fault code in a memory unit ifa monitored operating state of at least one of the plurality of clutchcontrol valves is different from a corresponding one of the expectedoperating states. Logging a fault code may comprise including in thefault code information indicative of a current one of a plurality ofdifferent operating modes of the transmission. Logging a fault code mayfurther comprise including in the fault code information identifyingwhich of the plurality of clutch control valves has a monitoredoperating state that is different from a corresponding expectedoperating state.

A fail-to-neutral diagnostic system for a transmission including avariator may comprise a plurality of clutch control valves each fluidlycoupled to a corresponding gear engagement clutch of the transmission, aplurality of pressure switches each fluidly coupled to a different oneof the plurality of clutch control valves, and a control circuit. Thecontrol circuit may include a memory having instructions stored thereinthat are executable by the control circuit to determine from theplurality of pressure switches actual operating states of each of theplurality of clutch control valves, to determine expected operatingstates of each of the plurality of clutch control valves based oncurrent operating conditions of the transmission, and to force thetransmission to a true neutral condition if an actual operating state ofat least one of the plurality of clutch control valves is different froma corresponding one of the expected operating states.

Each of the plurality of pressure switches may be configured to assumean operating state depending upon the operating state of a correspondingone of the plurality of clutch control valves. The instructions storedin the memory may include instructions that are executable by thecontrol circuit to determine from the plurality of pressure switchesactual operating states of each of the plurality of clutch controlvalves by monitoring operating states of each of the plurality ofpressure switches, and to determine expected operating states of each ofthe plurality of clutch control valves by determining expected operatingstates of each of the plurality of pressure switches. The transmissionmay have a plurality of operating modes each operable in a differentroad speed ranges of a vehicle carrying the transmission. Theinstructions stored in the memory may include instructions that areexecutable by the control circuit to determine expected operating statesof each of the plurality of clutch control valves by determining whichof the plurality of operating modes the transmission is currentlyoperating in, and then determining the expected operating states of eachof the plurality of clutch control valves for the current operating modeof the transmission based on the current operating conditions of thetransmission.

The instructions stored in memory may include instructions that areexecutable by the control circuit to log a fault code in the memory ifan actual operating state of at least one of the plurality of clutchcontrol valves is different from a corresponding one of the expectedoperating states. The instructions stored in memory may further includeinstructions that are executable by the control circuit to include inthe fault code information indicative of a current one of a plurality ofdifferent operating modes of the transmission. The instructions storedin memory may further include instructions that are executable by thecontrol circuit to include in the fault code information identifyingwhich of the plurality of clutch control valves has an actual operatingstate that is different from a corresponding expected operating state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one illustrative embodiment of a system forcontrolling operation of a toroidal traction drive automatictransmission including fail-to-neutral controls.

FIG. 2A is a diagram illustrating operation of one illustrativeembodiment of a variator that forms part of the toroidal traction driveautomatic transmission illustrated in FIG. 1.

FIG. 2B is a diagram further illustrating operation of the variator ofFIG. 2A.

FIG. 3 is a schematic diagram of one illustrative embodiment of theelectro-hydraulic control system that forms part of the toroidaltraction drive automatic transmission illustrated in FIG. 1.

FIG. 4 is a flowchart of one illustrative embodiment of a process formonitoring certain fault states of the electro-hydraulic control systemof FIG. 3 and commanding true neutral upon detection of at least some ofthe fault states.

FIG. 5 is a table of expected valve pressure states for the variousoperating modes of the transmission.

FIG. 6 is a magnified view of the fault valve illustrated in FIG. 3showing one operating state thereof.

FIG. 7 is another magnified view of the fault valve illustrated in FIG.3 showing an opposite operating state thereof.

FIG. 8 is a magnified view of the sign valve illustrated in FIG. 3showing one operating state thereof.

FIG. 9 is another magnified view of the sign valve illustrated in FIG. 3showing an opposite operating state thereof.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to a number of illustrativeembodiments shown in the attached drawings and specific language will beused to describe the same.

Referring now to FIG. 1, a block diagram is shown of one illustrativeembodiment of a system 10 for controlling operation of a toroidaltraction drive automatic transmission 14. In the illustrated embodiment,a power plant or energy center 12 is coupled to an automatictransmission 14 such that a rotatable output shaft 16 of the power plant12 is coupled to a rotatable input shaft 18 of the transmission 14 in aconventional manner. The input shaft 18 is coupled, in the illustratedembodiment, to a combination variator and gear set 20 that furtherincludes a plurality of selectively engageable friction devices, e.g.,one or more conventional, selectively engageable clutches or the like,and an output of the combination variator and gear set 20 is coupled toa rotatable output shaft 22. The combination variator and gear set 20 isillustratively controlled by an electro-hydraulic control system 24,some of the details of which will be described in greater detailhereinafter.

The power plant 12 is generally an apparatus that produces rotationaldrive power at the output shaft 16. Examples of the power plant 12include, but should not be limited to, one or any combination of a oneor more engines, such as an internal combustion engine of the sparkignited, compression ignition or other variety, a steam engine, or typeof engine that produces mechanical energy from one or more other fuelsources, one or more electrical generators, and the like.

The combination variator and gear set 20 illustratively includes aconventional full-toroidal, traction-drive variator that is coupled to aconventional gear set. Referring to FIGS. 2A and 2B, one illustrativeembodiment of some of the structural features of such a full-toroidal,traction-drive variator 40 is shown. In the illustrated embodiment, thevariator 40 includes a pair of opposing, toroidal-shaped disks 42 and 44that rotate independently of each other. For example, the disk 42 isrigidly coupled to the input shaft 18 of the transmission 14 such thatthe disk 42 is rotatably driven by the power plant 12. The disk 44 isrigidly coupled to an output shaft 46 of the variator 40, and isrotatably coupled to the shaft 18 such that the disk 44 rotates freelyabout the shaft 18. The output shaft 46 of the variator 40 is coupleddirectly, or indirectly through one or more transmission gears, to theoutput shaft 22 of the transmission 14 such that output shaft 46 of thevariator 40 drives one or more wheels of a vehicle (not shown) carryingthe power plant 12 and transmission 14.

A number of rollers 48 are illustratively positioned between opposinginner, arcuate-shaped surfaces of the disks 42 and 44, and a tractionfluid (not shown) is disposed between the rolling surface of each suchroller 48 and the inner surfaces of the disks 42 and 44. In theillustrated embodiment, the rolling surfaces of the various rollers 48therefore do not contact, in a structural sense, the inner surface ofeither disk 42, 44; rather torque is transmitted by the various rollers48 between the two disks 42, 44 via the traction fluid. It is becausetorque is transferred between the two disks 42, 44 via the tractionfluid and not via structural contact between the rolling surfaces of therollers 48 and the arcuate inner surfaces of the disks 42, 44 that thevariator is referred to as a traction-drive apparatus.

In the embodiment illustrated in FIGS. 2A and 2B, two such rollers 48 ₁and 48 ₂ are shown operatively positioned between the opposing innersurfaces of the two disks 42, 44. A roller actuator 50 ₁, e.g., in theform of a conventional hydraulically actuated piston, is coupled to theroller 48 ₁ via a bracket 52 ₁, and another roller actuator 50 ₂, e.g.,in the form of another conventional hydraulically actuated piston, iscoupled to the roller 48 ₂ via a bracket 52 ₂. It will be understoodthat the brackets 52 ₁ and 52 ₂ do not represent rotatable shafts aboutwhich the rollers 48 ₁ and 48 ₂ may be rotatably driven. Rather, thebrackets 52 ₁ and 52 ₂ represent structures about which the rollers 48 ₁and 48 ₂ rotate. In one actual implementation, for example, the brackets52 ₁ and 52 ₂ are configured to attach to the central hub of the rollers48 ₁ and 48 ₂ on either side thereof such that the brackets 52 ₁ and 52₂ and actuators 50 ₁ and 50 ₂ would extend generally perpendicular tothe page illustrating FIGS. 2A and 2B.

The hydraulically controlled actuators 50 ₁ and 50 ₂ are eachillustratively controllable, by selectively controlling a high-sidehydraulic pressure applied to one side of the actuator and a low-sidehydraulic pressure applied to the opposite side of the actuator, tothereby control torque transferred from a corresponding roller 48 ₁, 48₂ relative to the inner, annular surfaces of the two disks 42, 44. Theactuators 50 ₁ and 50 ₂ illustratively control driveline torque ratherthan the position or pitch of the rollers 48 ₁ and 48 ₂. The rollers 48₁ and 48 ₂ are free-castoring, and are responsive to the actuators 50 ₁and 50 ₂ to seek a position that provides the correct ratio match ofengine and drive train speeds based on input energy equaling outputenergy.

In one illustrative implementation, the variator 40 includes two sets ofdisks 42 and 44, with the pairs of the disks 42 rigidly coupled to eachother and with the pairs of the disks 44 also rigidly coupled to eachother, such that the embodiment illustrated in FIGS. 2A and 2Brepresents one-half of such an implementation. In this illustrativeimplementation, three rollers are positioned between each opposing setof disks 42, 44 for a total of six rollers 48 ₁-48 ₆ and sixcorresponding hydraulically controlled actuators 50 ₁-50 ₆. It will beunderstood, however, that this particular implementation of the variator40 is shown and described only by way of example, and that otherembodiments of the variator 40 that include more or fewer pairs of disks42, 44, that include more or fewer rollers 48 and hydraulicallycontrolled actuators 50, and/or that are configured to be only partiallytoroidal in shape, may alternatively be used. It will further beunderstood that while the operation of the variator 40 illustrated anddescribed herein as being generally hydraulically controlled, thisdisclosure contemplates embodiments in which operation of the variator40 is controlled via purely electronic or electro-mechanical structures.

Referring again to FIG. 1, the gear set within the combination variatorand gear set 20 illustratively includes one or more conventionalplanetary gear set(s) and/or other gear set(s) that define(s) at leasttwo automatically selectable gear ratios and that is coupled to, orintegrated with, the variator, e.g., the variator 40 illustrated anddescribed with respect to FIG. 2. The combination variator and gear set20 further illustratively includes a number of conventional frictiondevices, e.g., clutches, which may be selectively controlled to therebycontrol shifting of the transmission 14 between the two or more gearratios. In alternate embodiments, the gear set may include more than oneplanetary gear set, one or more planetary gear sets in combination withone or more other conventional gear sets, or exclusively one or morenon-planetary gear sets.

In the example embodiment illustrated in FIG. 1, the transmission 14includes three friction devices, e.g., in the form of three conventionalclutches C1, C2 and C3. In this embodiment, each clutch C1, C2 and C3 isoperated in a conventional manner by fluid pressure under the control ofthe electro-hydraulic control system 24. In this regard, a fluid path 25₁ is fluidly coupled between the electro-hydraulic control system 24 andthe clutch C1, a fluid path 25 ₂ is fluidly coupled between theelectro-hydraulic control system 24 and the clutch C2, and a fluid path25 ₃ is fluidly coupled between the electro-hydraulic control system 24and the clutch C3. The gear set and the clutches C1, C2 and C3 areillustratively arranged to provide four separate modes of operation ofthe transmission 14, and the various operating mode of the transmission14 are selectively controlled by the operation of the clutches C1, C2and C3.

In a first operating mode, M1, for example, the clutch C1 is applied,e.g., engaged, while the clutches C2 and C3 are released, e.g.,disengaged, and in this mode forward or reverse launch can beaccomplished, and the vehicle carrying the transmission 14 can beoperated at vehicle speeds up to about 10 miles per hour. In a secondoperating mode, M2, as another example, the clutch C2 is engaged whilethe clutches C1 and C3 are disengaged, and in this mode the vehicle canbe operated at vehicle speeds in the range of about 10-30 miles perhour. In a third operating mode, M3, as yet another example, the clutchC3 is engaged while the clutches C1 and C2 are disengaged, and in thismode the vehicle can be operated at vehicle speeds greater than about 30miles per hour. In a fourth mode, M0, as a final example, the clutchesC1, C2 and C3 are all disengaged, and in this mode the transmission 14is in so-called “true neutral.” In the transitional states between thevarious operating modes M1, M2 and M3, the variator torque isillustratively reversed to assist transitions from one operating mode tothe next.

The system 10 further includes a transmission control circuit 30 thatcontrols and manages the overall operation of the transmission 14. Thetransmission control circuit 30 includes a number, M, of operatingparameter inputs, OP₁-OP_(M), that are electrically connected tocorresponding operating parameter sensors included within theelectro-hydraulic control system 24 via corresponding signal paths 26₁-26 _(M), wherein M may be any positive integer. The one or moreoperating parameter sensors included within the electro-hydrauliccontrol system 24, examples of which will be described hereinafter,produce corresponding operating parameter signals on the signal paths 26₁-26 _(M), which are received by the transmission control circuit 30.The transmission 14 further includes a number, N, of electricallycontrollable actuators included within the electro-hydraulic controlsystem 24 that are each electrically connected to a different one of acorresponding number of actuator control outputs, AC₁-AC_(N) of thetransmission control circuit 30 via corresponding signal paths 28 ₁-28_(N), wherein N may be any positive integer. The one or moreelectrically controllable actuators included within theelectro-hydraulic control system 24, examples of which will be describedhereinafter, are responsive to actuator control signals produced by thetransmission control circuit 30 on the corresponding signal paths 28₁-28 _(N) to control various operational features of the transmission14.

Illustratively, the transmission control circuit 30 ismicroprocessor-based, and includes a memory unit 32 having instructionsstored therein that are executable by the control circuit 30 to controloperation of the transmission 14 generally, and more specifically tocontrol operation of the electro-hydraulic control system 24 as will bedescribed herein. It will be understood, however, that this disclosurecontemplates other embodiments in which the transmission control circuit30 is not microprocessor-based, but is configured to control operationof the transmission 14 generally and operation of the electro-hydraulicsystem 24 more specifically, based on one or more sets of hardwiredinstructions and/or software instructions stored in the memory unit 32.

Referring now to FIG. 3, a schematic diagram is shown of oneillustrative embodiment of the electro-hydraulic control system 24 ofFIG. 1. In the illustrated embodiment, the electro-hydraulic controlsystem 24 is roughly divided in two separate control sections; avariator control section 56 and a clutch control section 58. Aconventional fluid pump 60 is configured to supply transmission fluid,e.g., conventional transmission oil, to the variator control section 56from a source 64 of transmission fluid, e.g., a conventionaltransmission sump. In the illustrated embodiment, a fluid inlet of thefluid pump 60 fluidly coupled to the sump 64 via a fluid passageway 62.A fluid outlet of the pump 60 is fluidly coupled to an inlet of avariator main regulation block 66, and one of the output signal paths 28₆ of the control circuit 30 is electrically connected to the variatormain regulation block 66. The variator main regulation block 66 includesconventional components, e.g., one or more valves, responsive to acontrol signal produced on the signal path 28 ₆ by the transmissioncontrol circuit 30 to supply pressure-regulated transmission fluid at afluid outlet of the block 66 in a conventional manner.

The fluid outlet of the variator main regulation block 66 is fluidlycoupled to a fault valve 68 via a variator main fluid path 67 (VAM), andanother one of the output signal paths 28 ₁ of the control circuit 30 iselectrically connected to an electronic actuator 70, e.g., aconventional solenoid, of the fault valve 68. The fault valve 68 isconfigured to be responsive to a control signal produced on the signalpath 28 ₁ by the transmission control circuit 30 to control operation ofthe fault valve 68 between a stroked position and an unstroked positionas will be described in greater detail hereinafter. A variator fluidpath 72 is fluidly coupled between the fault valve 68 and fluid inletsof two separate variator trim valves 74 and 78. For purposes of thisdocument, the fluid path 72 may be referred to herein as a variatorfluid (VF) supply path.

The variator trim valves 74 and 78 each include an actuator 76 and 80respectively that is electrically connected to the transmission controlcircuit 30 via a signal path 28 ₂ and 28 ₃ respectively. A fluid outletof the variator trim valve 74 is fluidly coupled to a high-side fluidinlet of the variator control section 56 of the electro-hydrauliccontrol system 24 via a fluid path 82, and a fluid outlet of thevariator trim valve 78 is fluidly coupled to a low-side fluid inlet ofthe variator control section 56 of the electro-hydraulic control system24 via a fluid path 94. For purposes of this document, the fluid paths82 and 94 may be referred to herein as S1 and S2 respectively.

In the illustrated embodiment, the actuators 76 and 80 areillustratively conventional electronically actuated solenoids, and thetrim valves 74 and 78 are illustratively variable-bleed valves thatproduce variable-pressure transmission fluid at their fluid outletsbased on control signals produced by the transmission control circuit 30on the signal paths 28 ₂ and 28 ₃ respectively. The transmission controlcircuit 30 thus controls the fluid pressures within the S1 and S2 fluidpaths from the VF fluid path (connected to the fault valve 68) viaelectronic control of the trim valves 74 and 78 respectively.

The S1 fluid path (82) is fluidly coupled to one end of a conventionaldamper 84, an opposite end of which is fluidly coupled to a variatorhigh-side fluid passageway 86. In the embodiment illustrated in FIG. 3,the variator includes six actuators, 50 ₁-50 ₆, e.g., conventionalpistons, and the variator high-side fluid passageway 86 is fluidlycoupled to one side, e.g., a high side, of each such actuator 50 ₁-50 ₆via a corresponding conventional damper 88 ₁-88 ₆. A conventional checkvalve 85 is interposed between the variator high-side fluid passageway86 and a control main (COM) fluid path 104, and another conventionalcheck valve 87 is interposed between the variator high-side fluidpassageway 86 and an endload fluid passageway 90.

The S2 fluid path (94) is similarly fluidly coupled to one end ofanother conventional damper 96, an opposite end of which is fluidlycoupled to a variator low-side fluid passageway 98. The variatorlow-side fluid passageway 98 is fluidly coupled to an opposite side,e.g., a low side, of each actuator 50 ₁-50 ₆ of the variator via acorresponding conventional damper 100 ₁-100 ₆. A conventional checkvalve 89 is interposed between the variator low-side fluid passageway 98and the control main (COM) fluid path 104, and another conventionalcheck valve 102 is interposed between the variator low-side fluidpassageway 98 and the endload fluid passageway 90. The endload fluidpassageway 90 is fluidly coupled to an endload relief valve 92, which isfurther fluidly coupled between the high side and the low side of theactuator 50 ₆. Further details relating to one illustrative structureand method of operating the endload relief valve 92 are provided inco-pending U.S. Patent Application Ser. No. 61/287,020, having AttorneyDocket No. 46582-209632 (ATP-0047-USP), the disclosure of which isincorporated herein by reference in its entirety.

Another conventional fluid pump 106 is configured to supply transmissionfluid from the sump 64 to the clutch control section 58 of theelectro-hydraulic control system 24. In the illustrated embodiment, afluid inlet of the fluid pump 106 fluidly coupled to the sump 64 via thefluid passageway 62, and fluid outlet of the pump 106 is fluidly coupledto the fault valve 68 and to a fluid inlet of a clutch and control mainregulation, cooler and lube block 110 via a fluid passageway 108.Another one of the output signal paths 28 ₇ of the control circuit 30 iselectrically connected to the clutch and control main regulation, coolerand lube block 110. The clutch and control main regulation, cooler andlube block 110 includes conventional components, e.g., one or morevalves, responsive to a control signal produced on the signal path 28 ₇by the transmission control circuit 30 to supply pressure-regulatedtransmission fluid to a clutch main, CLM, fluid passageway 108 and to acontrol main, COM, fluid passageway 104 in a conventional manner. Thecontrol main fluid passageway 104 is further fluidly coupled to thefault valve 68. An exhaust backfill valve 113 establishes an exhaustbackfill pressure, EB, in an exhaust backfill fluid passageway 116 thatis also fluidly coupled to the clutch and control main regulation,cooler and lube block 110 and to the fault valve 68. The clutch andcontrol main regulation, cooler and lube block 110 further includesconventional components for cooling and filtering the transmission fluidand for providing lubrication paths to the variator and to the variousgears of the gear set.

The clutch main fluid passageway, CLM, is fluidly coupled to the faultvalve 68, and a clutch fluid, CF, passageway 118 is fluidly coupledbetween the fault valve 68 and pair of clutch trim valves 120 and 122.Generally, the fault valve 68 is configured to be responsive to acontrol signal produced on the signal path 28 ₁ by the transmissioncontrol circuit 30 to control operation of the fault valve 68 toselectively supply and inhibit transmission fluid operating pressure tothe variator control section 56 and to the clutch control section 58 ofthe electro-hydraulic control system 24 as will be described in detailhereinafter.

The clutch trim valves 120 and 122 each illustratively include anelectronic actuator, e.g., an electrically controlled solenoid, 126 and128 respectively that is electrically connected to the transmissioncontrol circuit 30 via a signal path 28 ₄ and 28 ₅ respectively. A fluidinlet of each clutch trim valve 120 and 122 is fluidly coupled to thecontrol main, COM, fluid passageway 104. Each valve 120 and 122 isresponsive to a control signal produced by the transmission controlcircuit 30 on the signal path 28 ₄ and 28 ₅ respectively to selectivelyapply or block the control main pressure, COM, to a control end of aspool 125 and 127 respectively carried by each valve 120 and 122. Eachclutch trim valve 120 and 122 is further fluidly coupled to the exhaustbackfill pressure, EB, passageway 116 and to the clutch fluid, CF,passageway 118. The clutch trim valve 122 is further fluidly coupled tothe C2 clutch fluid path 25 ₂, and the clutch trim valve 122 isillustratively responsive to a control signal supplied by thetransmission control circuit 30 to the actuator 128 on the signal path28 ₅ to selectively activate, i.e., engage, and deactivate, i.e.,disengage, the clutch C2 via the clutch fluid path 25 ₂. For example, inthe diagram illustrated in FIG. 3, the clutch trim valve 122 is shown inits deactivated state in which the control end of the spool 127 fed bythe actuator 128 is exhausted such that the spool 127 of the valve 122fluidly couples the clutch fluid path 25 ₂ to the exhaust backfill, EB,pressure passageway 116, thereby deactivating or disengaging the clutchC2. In its activated state, the actuator 128 would feed the control mainpressure, COM, to the control end of the spool 127 thereby causing thespool 127 to move downwardly (in FIG. 3) and fluidly couple the clutchfluid path 25 ₂ to the clutch fluid, CF, passageway 118, therebyactivating or engaging the clutch C2.

The clutch trim valve 120 is further fluidly coupled to a clutchmultiplex valve 124 via a fluid passageway 132. The clutch multiplexvalve 124 illustratively includes an electronic actuator, e.g., anelectrically controlled solenoid, 134 that is electrically connected tothe transmission control circuit 30 via a signal path 28 ₆. A fluidinlet of the clutch multiplex valve 124 is fluidly coupled to thecontrol main, COM, fluid passageway 104, and the clutch multiplex valve124 is responsive to a control signal produced by the transmissioncontrol circuit 30 on the signal path 28 ₆ to selectively apply or blockthe control main pressure, COM, to a control end of a spool 135 carriedby the valve 124. The clutch multiplex valve 124 is further fluidlycoupled to the exhaust backfill pressure, EB, passageway 116 and to theC1 clutch fluid path 25 ₁ and the C3 fluid path 25 ₃.

The clutch trim valve 120 is illustratively responsive to a controlsignal supplied by the transmission control circuit 30 to the actuator126 on the signal path 28 ₄ to selectively supply clutch fluid, CF, fromthe clutch fluid passageway 118 or exhaust backfill pressure, EB, fromthe exhaust backfill passageway 116 to the clutch multiplex valve 124.For example, in the diagram illustrated in FIG. 3, the clutch trim valve120 is shown in its activated state in which the control end of thespool 125 fed by the actuator 126 is fed by the control main pressure,COM, such that the spool 125 of the valve 120 fluidly couples the fluidpath 132 to the clutch fluid, CF, fluid passageway 118, therebysupplying clutch fluid pressure, CF, to the clutch multiplex valve 124.In its deactivated state, the actuator 126 would exhaust the control endof the spool 125 thereby causing the spool 125 to move upwardly (in FIG.3) and fluidly couple the fluid passageway 132 to the exhaust backfill,EB, fluid passageway 116, thereby supplying exhaust backfill pressure,EB, to the clutch multiplex valve 124.

The clutch multiplex valve 124 is illustratively responsive to a controlsignal supplied by the transmission control circuit 30 to the actuator134 on the signal path 28 ₆ to selectively couple the fluid passageway132 to either the C1 clutch fluid path 25 ₁ or the C3 clutch fluid path25 ₃, to thereby activate, i.e., engage, and deactivate, i.e.,disengage, the clutch C1 via the clutch fluid path 25 ₁ or the clutch C3via the clutch fluid path 25 ₃. For example, in the diagram illustratedin FIG. 3, the clutch multiplex valve 124 is shown in its deactivatedstate in which the control end of the spool 135 fed by the actuator 134is exhausted such that the spool 135 of the valve 124 fluidly couplesthe C1 clutch fluid path 25 ₁ to the fluid path 132. If the trim valve120 is in its activated position as illustrated in FIG. 3, the C1 clutchfluid path 25 ₁ will thus be fluidly coupled to the clutch fluid, CF,passageway 118, thereby activating or engaging the C1 clutch. At thesame time, the valve 124 fluidly couples the C3 clutch fluid path 25 ₃to the exhaust backfill, EB, fluid passageway 116 to thereby deactivateor disengage the C3 clutch. If, on the other hand, the trim valve 120 isin its deactivated position, the exhaust backfill, EB, pressurepassageway 116, will be fluidly coupled to the C1 clutch fluid path 25 ₁and to the C3 clutch fluid path 25 ₃, thereby deactivating ordisengaging both of the clutches C1 and C3.

If instead the clutch multiplex valve 124 is in its activated state inwhich the control end of the spool 135 fed by the actuator 134 isfluidly coupled to the control main, COM, fluid passageway 104, thespool 135 of the valve 124 fluidly couples the C3 clutch fluid path 25 ₃to the fluid path 132. If the trim valve 120 is in its activatedposition as illustrated in FIG. 3, the C3 clutch fluid path 25 ₃ wouldthus be fluidly coupled to the clutch fluid, CF, passageway 118, therebyactivating or engaging the C3 clutch, and at the same time, the valve124 would fluidly couple the C1 clutch fluid path 25 ₁ to the exhaustbackfill, EB, fluid passageway 116 to thereby deactivate or disengagethe C1 clutch. If, on the other hand, the trim valve 120 is in itsdeactivated position, the exhaust backfill, EB, pressure passageway 116,will be fluidly coupled to the C1 clutch fluid path 25 ₁ and to the C3clutch fluid path 25 ₃, thereby deactivating or disengaging both of theclutches C1 and C3.

In the embodiment illustrated in FIG. 3, the variator section 156 of theelectro-hydraulic control system 24 further includes a pressuredifferential valve 150 including a spool 154 that is fluidly coupled atone end to the S1 fluid passageway 82, i.e., the high-side pressure ofthe variator actuators 50 ₁-50 ₆, and at its opposite end to the S2fluid passageway 94, i.e., the low-side pressure of the variatoractuators 50 ₁-50 ₆. The pressure differential valve 150 is furtherfluidly coupled to the control main, COM, fluid passageway 104.

In the illustrated embodiment, sensors are operatively positionedrelative to each of the valves 68, 120, 122, 124 and 150 to enablemonitoring of the operating states of each of these valves 68, 120, 122,124 and 150. In one illustrative embodiment, the sensors are provided inthe form of conventional pressure switches, although it will beunderstood that a conventional pressure sensor may be substituted forany one or more of the pressure switches. In any case, each of thepressure switches is electrically connected to the transmission controlcircuit 30 to allow monitoring by the transmission control circuit 30 ofthe states of the pressure switches and thus the operating states of thevalves 68, 120, 122, 124 and 150. In the embodiment illustrated in FIG.3, for example, a pressure switch 75 is fluidly coupled to the faultvalve 68, and is electrically connected to the transmission controlcircuit 30 via one of the signal paths 26 ₁. Another pressure switch 130is fluidly coupled to the trim valve 120, and is electrically connectedto the transmission control circuit 30 via one of the signal paths 26 ₂.Still another pressure switch 136 is fluidly coupled to the multiplexvalve 124, and is electrically connected to the transmission controlcircuit 30 via one of the signal paths 26 ₃. Yet another pressure switch142 is fluidly coupled to the trim valve 122, and is electricallyconnected to the transmission control circuit 30 via one of the signalpaths 26 ₄. Another pressure switch 152 is fluidly coupled to thepressure differential valve 150, and is electrically connected to thetransmission control circuit 30 via one of the signal paths 26 ₅. Thetransmission control circuit 30 is operable to process the signalsproduced by the pressure switch 75, 130, 136, 142 and 152 in a knownmanner to determine corresponding operating states, i.e., whetheractivated or deactivated, of the various valves 68, 120, 122, 124 and150.

Referring now to FIG. 4, a flowchart is shown of one illustrativeembodiment of a process 160 for monitoring the statuses of the variouspressure switches 75, 130,136, 142 and 152 and commanding true neutralupon detection of certain pressure switch failures or faults, andlogging or storing any detected pressure switch failure or fault. Theterm “true neutral” is defined for purposes of this document as anoperating condition of the transmission in which none of the clutchesC1-C3 is activated or engaged, and corresponds to a transmissionoperating mode M0 identified and described hereinabove. The process 160is illustratively stored in the memory 32 of the transmission controlcircuit 30 in the form of instructions that are executable by thetransmission control circuit 30 to carry out the operations of theprocess 160 as will be described in detail hereinafter.

The process 160 begins at step 162, and thereafter at step 164 thetransmission control circuit 30 is operable to determine the presentoperating mode, OM, of the transmission 14. Generally, as describedhereinabove, the transmission 14 is operable in one of three non-neutraloperating modes, M1-M3, and the operating mode of the transmission 14 atany given instant in time will depend primarily on the road speed of thevehicle carrying the transmission 14. Because the transmission controlcircuit 30 controls and manages the overall operation of thetransmission 14, the transmission control circuit 30 will generally haveknowledge at any given time of the current operating mode, OM, of thetransmission 14. The transmission control circuit 30 thus executes step164 by determining internally the current operating mode, OM, of thetransmission 14. Thereafter at step 166, the transmission controlcircuit 30 is operable to determine the status of each pressure switch75, 130, 136, 142 and 152, e.g., by sampling and processing the pressureswitch signals produced by the pressure switches on the correspondingsignal paths 26 ₁-26 ₅.

Following step 166, the process 160 advances to step 168 where thetransmission control circuit 30 is operable to compare the status of thepressure switches, determined at step 166, with expected pressure switchstatuses based on the current operating mode of the transmission 14.Illustratively, a table or other form of expected pressure switch statusvalues by transmission operating mode is stored in the memory 32, andthe transmission control circuit 30 executes step 168 by comparing thepressure switch statuses determined at step 166 with such a table orother form of expected pressure switch status values stored in thememory 32. Referring to FIG. 5, an example is shown of one such expectedpressure switch status table 190 that lists expected pressure switchstatus values for each of the three operating modes of the transmission.

Returning again to FIG. 4, the process 160 advances from step 168 tostep 170 where the transmission control circuit 30 determines based onthe comparison of step 168 whether a fault or failure associated withthe fault valve 68 is detected. As illustrated in the pressure switchstatus table 190 of FIG. 5, the pressure switch 75 is expected to have avalue of “1” for each of the three operating modes, M1-M3, of thetransmission 14. Thus, if the comparison at step 168 reveals that theactual value of the pressure switch 75 is “1,” then the transmissioncontrol circuit 30 determines at step 170 that no fault valve fault isdetected, and the process 160 advances to step 172. Otherwise, thetransmission control circuit 30 determines at step 170 that a faultvalve fault exists and the process 160 advances to step 178 where thetransmission control circuit 30 stores or logs an appropriate fault codein the memory 32.

At step 172 the transmission control circuit 30 determines based on thecomparison of step 168 whether a fault or failure associated with theclutch valves 120, 122 and 124 is detected. As illustrated in thepressure switch status table 190 of FIG. 5, the pressure switches 136,130 and 142 are expected to have the values “0 1 0” respectively formode 1, the values “0 0 1” respectively for mode 2 and the values “1 10” respectively for mode 3. If the comparison at step 168 reveals thatthe actual values of the pressure switches 136, 130 and 142 match theexpected values for the current operating mode of the transmission 14,then the transmission control circuit 30 determines at step 172 that noclutch valve fault is detected, and the process 160 advances to step174. Otherwise, the transmission control circuit 30 determines at step172 that a clutch valve fault exists and the process 160 advances tostep 176 where the transmission control circuit 30 commands trueneutral. Illustratively, the transmission control circuit 30 is operableto command true neutral by deactivating or destroking the fault valve68, thereby forcing the transmission 14 to a true neutral condition ifat least one of the actual values of the pressure switches 136, 130 and142 is different from a corresponding one of the expected values of thepressure switches 136, 130 and 142.

Referring now to FIGS. 6 and 7, magnified views of the fault valve 68are shown in which FIG. 6 illustrates the fault valve 68 in itsactivated or stroked position (as also illustrated in FIG. 3), and inwhich FIG. 7 illustrates the fault valve 68 in its deactivated orunstroked position. In its activated or stroked position illustrated inFIG. 6, the spool 200 is forced by the control main pressure, COM,applied to its control end 202 to be positioned within the fault valve68 to cause the variator main pressure, VAM, to be fluidly coupled tothe variator fluid (VF) passageway 116, to cause the clutch mainpressure, CLM, to be fluidly coupled to the clutch fluid, CF, passageway118 and to cause the control main pressure, COM, to be fluidly coupledto the pressure switch 75 such that the pressure switch 75 produces alogic high signal, or “1.” In this normally activated or strokedposition, the fault valve 68 thus feeds the variator main pressure, VAM,to the two variator trim valves 74 and 78 via the fluid passageway 72,and feeds the clutch main pressure, CLM, to the two clutch trim valves120 and 122 via the fluid passageway 118 (see FIG. 3).

In its deactivated or unstroked position illustrated in FIG. 7, thecontrol end 202 of the spool is exhausted and the spool 200 is therebyforced to be positioned within the fault valve 68 to cause the controlmain pressure, COM, to be fluidly coupled to the variator fluid (VF)passageway 116, to cause the exhaust backfill pressure, EB, to befluidly coupled to the clutch fluid, CF, passageway 118 and to cause theexhaust backfill pressure, EB, to also be fluidly coupled to thepressure switch 75 such that the pressure switch 75 produces a logic lowsignal, or “0.” In this deactivated or unstroked position, the faultvalve 68 thus feeds the control main pressure, COM, to the two variatortrim valves 74 and 78 via the fluid passageway 72, and feeds the exhaustbackfill pressure, EB, to the two clutch trim valves 120 and 122 via thefluid passageways 116 and 118. Referring again to FIG. 3, with theexhaust backfill pressure, EB, supplied to the fluid path 118, none ofthe clutches C1-C3 can be activated or engaged regardless of theoperating states of any of the clutch valves 120, 122 and 124, and thetransmission 14 is thereby in the true neutral state.

Referring again to FIG. 4, the process 160 advances from step 176 tostep 178 where the transmission control circuit 30 stores or logs anappropriate fault code in the memory 32, i.e., a fault code appropriatefor a clutch valve fault or failure. The stored or logged fault code mayinclude information only as to the existence of a clutch valve fault orfailure, information relating to which transmission operating mode wasactive, i.e., mode 1, 2 or 3, when the clutch valve fault or failure wasdetected, information relating to the specific pressure switch for whichthe fault or failure was detected, and/or whether the clutch valvefailure or fault occurred because the monitored state of the pressureswitch in question was a “1” when a “0” was expected or vice versa.

At step 174 the transmission control circuit 30 determines, based on thecomparison of step 168, whether a fault or failure associated with thepressure differential valve 150 is detected. As illustrated in thepressure switch status table 190 of FIG. 5, the pressure switch 152 mayhave a value of “1” or “0” in each of the three operating modes.Referring now to FIGS. 8 and 9, magnified views of the pressuredifferential valve 150 are shown illustrating the two opposite states ofthe pressure switch 152. In FIG. 8, for example, which is also the stateof the valve 150 illustrated in FIG. 3, the spool 154 is drivenupwardly, indicating that the pressure S2 is greater than that of S1,which means that the low-side pressure applied to the variator actuators50 ₁-50 ₂ via the fluid passageway 98 is greater than the high-sidepressure applied to the variator actuators 50 ₁-50 ₆ via the fluidpassageway 86. In this position, the spool 154 fluidly couples thepressure switch 152 to exhaust, and in this position the pressure switch152 thus produces a logic low signal or “0.” In FIG. 9, in contrast, thespool 154 is driven downwardly, indicating that the pressure S1 isgreater than that of S2, which means that the high-side pressure appliedto the variator actuators 50 ₁-50 ₆ via the fluid passageway 86 isgreater than the low-side pressure applied to the low-side pressureapplied to the variator actuators 50 ₁-50 ₆ via the fluid passageway 98.In this position, the spool 154 fluidly couples the pressure switch 152to the control main pressure, COM, and in this position the pressureswitch 152 thus produces a logic high signal or “1.” Illustratively, thepressure differential valve 150 is designed to have a specified amountof hysteresis between the switching states of the pressure switch 152,and in one embodiment the hysteresis is approximately 15-20% such thatthe differential pressure between S1 and S2 must be greater than about15-20% before the pressure switch 152 changes state. Those skilled inthe art will appreciate that this hysteresis value is provided only byway of example and that other hysteresis values, or no hysteresis value,may alternatively be used.

As with the operating modes, M1-M3, the transmission control circuit 30likewise controls and manages the operation of the variator, and thetransmission control circuit 30 thus has knowledge of whether theoperating torque of the variator is positive or negative at any giveninstant in time based on current transmission operating conditions.Illustratively, a positive operating torque of the variator occurs whenthe differential pressure valve 150 is positioned such that the pressureswitch 152 produces a “1,” e.g., as illustrated in FIG. 9, and anegative operating torque of the variator occurs when the differentialpressure valve 150 is positioned such that the pressure switch 152produces a “0,” e.g., as illustrated in FIG. 8, although the pressuredifferential valve may alternatively be designed such that the pressureswitch 152 produces a “1” during periods of negative variator torque andproduces a “0” during periods of positive variator torque. Thetransmission control circuit 30 can thus determine from the state of thepressure differential valve, e.g., from the state of the pressure switch152, a variator torque sign corresponding to whether torque transferredby the at least one roller to the variator is positive or negative.Because the transmission control circuit 30 has knowledge of whether theoperating torque of the variator is positive or negative, thetransmission control circuit 30 accordingly has knowledge of theexpected value of the pressure switch 152 at any instant in time. If, atstep 168, the comparison reveals that the actual value of the pressureswitch 152 matches the expected value of the pressure switch 152, thenthe transmission control circuit 30 determines at step 174 that nopressure differential valve fault is detected, and the process 160 loopsback to step 164. Otherwise, the transmission control circuit 30determines at step 174 that a pressure differential valve fault existsand the process 160 advances to step 176 where the transmission controlcircuit 30 commands true neutral as described hereinabove. Thereafter atstep 178, the transmission control circuit 30 stores or logs a faultcode corresponding to a differential pressure valve fault or failure.The stored or logged fault code may include information only as to theexistence of a differential pressure valve fault or failure, and/or mayinclude information relating to whether the differential pressure valvefault or failure occurred because the monitored state of the pressureswitch 152 was a “1” when a “0” was expected or vice versa.

While the invention has been illustrated and described in detail in theforegoing drawings and description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly illustrative embodiments thereof have been shown and described andthat all changes and modifications that come within the spirit of theinvention are desired to be protected.

1. A fail-to-neutral diagnostic method for a transmission including avariator, the method comprising: monitoring a state of a pressuredifferential valve fluidly coupled to a high side pressure applied to atleast one actuator coupled to at least one corresponding roller of thevariator and also fluidly coupled to a low side pressure applied to theat least one actuator; determining from the state of the pressuredifferential valve a variator torque sign corresponding to whethertorque transferred by the at least one roller is positive or negative;determining an expected variator torque sign based on current operatingconditions of the transmission; and commanding the transmission to atrue neutral condition if the determined variator torque sign isdifferent from the expected variator torque sign.
 2. The method of claim1 wherein the pressure differential valve comprises a spool having oneend fluidly coupled to the high-side pressure and an opposite endfluidly coupled to the low-side pressure, and wherein the variatortorque sign has one value when the high-side pressure is sufficientlygreater than the low-side pressure and has an opposite value when thelow-side pressure is sufficiently greater than the high-side pressure,and wherein determining from the state of the pressure differentialvalve a variator torque sign comprises determining the variator torquesign based on a position of the spool relative to the pressuredifferential valve.
 3. The method of claim 2 wherein the transmissionfurther includes a pressure switch fluidly coupled to the pressuredifferential valve, the pressure switch configured to assume one statewhen the high-side pressure is sufficiently greater than the low-sidepressure to cause the spool to move to one extreme relative to thepressure differential valve, and to assume an opposite state when thelow-side pressure is sufficiently greater than the high-side pressure tocause the spool to move to an opposite extreme relative to the pressuredifferential valve, and wherein determining from the state of thepressure differential valve a variator torque sign comprises determiningthe variator torque sign from the state of the pressure switch, thevariator torque sign having one value when the pressure switch hasassumed the one state and having an opposite value when the pressureswitch has assumed the opposite state.
 4. The method of claim 3 whereinthe expected variator torque sign has the one value if the pressureswitch is expected to be in the one state, and has the opposite value ifthe pressure switch is expected to be in the opposite state, and whereincommanding the transmission to a true neutral condition comprisescommanding the transmission to the true neutral condition if thevariator torque sign is the one value and the expected variator torquesign is the opposite value, and if the variator torque sign is theopposite value and the expected variator torque sign is the one value.5. The method of claim 4, wherein the one value is a positive value andthe opposite value is a negative value.
 6. The method of claim 2,wherein the one value is a positive value and the opposite value is anegative value.
 7. The method of claim 3, wherein an amount ofhysteresis is associated with the pressure switch, and wherein thepressure switch assumes the one state when the high-side pressure isgreater than the low-side pressure by the amount of hysteresis.
 8. Themethod of claim 7, wherein the amount of hysteresis is at least 15% andno greater than 20%.
 9. The method of claim 3, wherein an amount ofhysteresis is associated with the pressure switch, and wherein thepressure switch assumes the opposite state when the low-side pressure isgreater than the high-side pressure by the amount of hysteresis.
 10. Themethod of claim 9, wherein the amount of hysteresis is at least 15% andno greater than 20%.
 11. The method of claim 1 further comprisinglogging a fault code in a memory unit if the determined variator torquesign is different from the expected variator torque sign.
 12. Afail-to-neutral diagnostic system for a transmission including avariator, the system comprising: a pressure differential valve fluidlycoupled to a high side pressure applied to at least one actuator coupledto the variator and also fluidly coupled to a low side pressure appliedto the at least one actuator; a pressure switch fluidly coupled to thepressure differential valve and configured to assume one of two oppositestates depending upon which of the high-side and low-side pressures isgreater than the other; and a control circuit including a memory havinginstructions stored therein that are executable by the control circuitto determine from current operating conditions of the transmission anexpected state of the pressure switch, to determine an actual state ofthe pressure switch and to command the transmission to a true neutralcondition if the actual state of the pressure is different from theexpected state of the pressure switch.
 13. The system of claim 12,wherein the pressure switch is configured to assume one state if thehigh-side pressure is sufficiently greater than the low-side pressureand to assume an opposite state if the low-side pressure is sufficientlygreater than the high-side pressure.
 14. The system of claim 13 whereinthe instructions stored in the memory include instructions that areexecutable by the control circuit to determine that the expected stateof the pressure switch is the one state if the high-side pressure isexpected to be sufficiently greater than the low-side pressure and isthe opposite state if the low-side pressure is expected to besufficiently greater than the high-side pressure; and wherein theinstructions stored in the memory include instructions that areexecutable by the control circuit to command the neutral state if thepressure switch has assumed the one state and the expected state of thepressure switch is the opposite state, and to command the neutral stateif the pressure switch has assumed the opposite state and the expectedstate of the pressure switch is the one state.
 15. The system of claim12 wherein the instructions stored in the memory include instructionsthat are executable by the control circuit to log a fault code in amemory unit if the determined actual state of the pressure switch isdifferent from the expected state of the pressure switch.
 16. The systemof claim 12, wherein the expected state of the pressure switch isdetermined from a table stored in the memory defining the expected stateof the pressure switch based on transmission operating mode.
 17. Thesystem of claim 13, wherein the pressure differential valve determines avariator torque sign corresponding to whether torque transferred by thevariator is positive or negative, (ii) the variator torque sign ispositive when the high-side pressure is sufficiently greater than thelow-side pressure, and (iii) the one state of the pressure switchcorresponds to the positive variator torque sign.
 18. The system ofclaim 17, wherein the variator torque sign is negative when the low-sidepressure is sufficiently greater than the high-side pressure, andwherein the opposite state of the pressure switch corresponds to thenegative variator torque sign.